Recently, great attention is paid to organopolysiloxanes because of their excellent properties including light transmission, heat resistance, gas permeability and chemical stability. Since organopolysiloxanes having any desired properties can be prepared by changing the type of monomers, charge monomer composition, and reaction conditions in their synthesis process, they have found practical use in a variety of fields.
Among others, an organopolysiloxane having a plurality of hydrosilyl groups (i.e., Si—H groups) in a common molecule is regarded essential as the crosslinker in addition curable organopolysiloxane resin compositions. A variety of multifunctional hydrosilyl-containing organopolysiloxanes have been developed. For example, Patent Document 1 discloses that a silicone resin is cured using an organopolysiloxane having two or four terminal hydrosiloxy groups (HR2SiO1/2 units) as the crosslinker. The cured resin is used as encapsulant over optical semiconductor devices. Patent Document 2 describes a cured product of an organopolysiloxane having a plurality of pendant hydrosiloxy groups (HRSiO2/2 units).
Patent Document 3 discloses that an organopolysiloxane having a hydrosilyl group may be bonded with a compound having an unsaturated hydrocarbon group by hydrosilylation reaction. The resulting product is useful as a starting material to organically modified silicones such as epoxy- and acrylic-modified silicones.
These hydrosilyl-containing organopolysiloxanes are useful materials. As is well known in the art, the hydrosilyl groups are divided into three types: HR2SiO1/2, HRSiO2/2, and HSiO3/2. Among these, the organopolysiloxanes having HR2SiO1/2 (terminal hydrosiloxy group) are highly reactive in hydrosilylation. This leads to an advantage in productivity when they are used as crosslinker or starting material to modified silicones. Nowadays, the terminal hydrosilyl-containing organopolysiloxanes are advantageously used.
One general method for preparing a multifunctional hydrosilyl-containing organopolysiloxane involves contacting a chlorosilane and/or alkoxysilane with a stoichiometric amount of water in an organic solvent in the presence of an acid catalyst, to induce hydrolysis and condensation reaction. However, when HR2SiX, R2SiX2, and RSiX3 or SiX4 (wherein X is a hydrolyzable group) are subjected to cohydrolytic condensation, the desired organopolysiloxane is not always obtained because random reactions take place. On cohydrolytic condensation of silanes wherein R and X are different, the desired organopolysiloxane is not obtained because reaction products are unbalanced due to differential reactivity.
For the synthesis of the desired organopolysiloxane, Patent Document 4 discloses the preparation of an organo-silicone condensation product by condensation of a silanol-containing siloxane in the presence of a catalyst which is a sodium or potassium salt of boric acid or phosphoric acid. Patent Document 5 describes the preparation of an organopolysiloxane by reaction of a silanol-containing siloxane in the presence of a catalyst which is selected from among a hydroxide, chloride, and oxide of an alkali or alkaline earth metal, and a basic metal salt. Patent Document 6 describes that even magnesium or calcium hydroxide can catalyze condensation reaction of a silanol-containing siloxane and an alkoxysilane as long as a protic solvent is co-present. Patent Document 7 describes the preparation of an organo-silicone condensate by reaction of a silicon compound having a silanol group and/or alkoxysilyl group in the presence of a catalyst selected from among strontium oxide, barium oxide, strontium hydroxide, barium hydroxide, and mixtures thereof.
With the methods of Patent Documents 4 to 7, organopolysiloxanes of controlled structure are obtainable by limiting reaction sites to silanol and alkoxy groups. Since the catalyst used is a solid catalyst, advantageously the catalyst can be readily removed from the organopolysiloxane product by filtration. These benefits are advantageous in the field where precise control of material is necessary or residual impurities are not permissible, for example, in the fields of optical, electronic, and medical materials.
As alluded to previously, organopolysiloxanes find practical use in a variety of fields by virtue of their useful properties. Among others, the organopolysiloxanes having a plurality of terminal hydrosilyl groups in the molecule are used in the fields where liquid silicone resins serving as semiconductor encapsulant or sealant must be crosslinked and cured. In these fields, since the physical properties (e.g., hardness and elongation) of a cured product must be tailored so as to comply with an ambient situation on use, a variety of hydrogenorganopolysiloxanes have been developed. The synthesis process is restricted in that the hydrosilyl group is susceptible to hydrolysis, i.e., dehydrogenation if a strong alkali or similar catalyst is present. For this reason or other, no satisfactory organopolysiloxane is available up to the present.
Meanwhile, addition curable silicone resin compositions have heretofore been used as the encapsulant for semiconductor devices such as LED, because of their good properties including heat resistance, light resistance and fast cure. For example, Patent Document 8 discloses an addition curable silicone resin composition exhibiting a high bonding force to LED packages of thermoplastic resins, typically PPA. Patent Document 9 discloses an addition curable silicone resin composition, with which optoelectronic chips are encapsulated by compression molding.
Although addition curable silicone resin compositions are commonly used as the semiconductor encapsulating material, their properties are still unsatisfactory. Particularly in the semiconductor encapsulant field, it is a common practice to encapsulate semiconductor devices by compression molding or transfer molding. From the aspect of productivity, there is a demand for a thermosetting silicone resin composition which is rapidly cured to a sufficient hardness for mold release. In prior art addition curable silicone resin compositions, an organohydrogenpolysiloxane having multifunctional SiH groups capable of 3D crosslinking in a common molecule is used as the crosslinker for enhancing the cure rate. However, such organohydrogenpolysiloxanes having a multiplicity of terminal SiH groups to provide an accelerated cure rate are difficult to synthesize while the type thereof is limited. Instead, an organohydrogenpolysiloxane having incorporated in its structure pendant SiH groups to provide a lower cure rate is often used as the crosslinker.
As pointed out above, Patent Documents 1 and 2 disclose organohydrogenpolysiloxanes. The organohydrogenpolysiloxane described in Patent Document 1 as having four terminal SiH groups provides an insufficient cure rate. The organohydrogenpolysiloxane described in Patent Document 2 as consisting of terminal SiH units and SiO4/2 units has the problem that the cured resin product is brittle and susceptible to cracks.